For every 1 liter of ghee, there are 905 grams. For every 1 liter of butter, there are 911 grams. The liter is the same, so that's your unit of volume. For every n units of volume, that ratio stays the same. ie, it's just 905g/911b. If you want to change that, then you'd have 905g/911b = xg/yb. For every x value, there's a unique y value, and vice versa.